1. Field of the Invention
This invention relates to medical instrumentation and a method and system for the use thereof and, more particularly, to electrocardiographic and cardiocirculatory monitoring equipment and a method and system relating thereto.
2. Prior Art
Every muscle can perform only one movement, the shortening of its fibers by contraction. This also applies to the heart muscle. Every action of a muscle has associated with it an electrical activity which changes in the course of the contraction. The electrical signal thus associated with the muscle action is transmitted through various tissues and ultimately reaches the surface of the body whereupon such electrical signals can be detected by electrodes applied to the skin. Thus, such signals that are being detected by the electrodes can be recorded with the aid of suitable electrocardiographic equipment or can be observed in or recorded with a monitor/recording unit. The record thus obtained is called an electrocardiogram or a rhythm-monitoring strip.
As early as 1855 action currents from the heart were recorded as measurements were being made of a beating frog heart. The first actual recording of a frog electrocardiogram was made by A. D. Waller in 1887. The first recording of a human heart electrical action signal (hereinafter "heart-signal") was made by A. D. Waller in 1889. Modern electrocardiography, however, started with Einthoven (credited with the bipolar lead triangle setting for recordings of standard limb leads I, II and III), who invented the string galvanometer and applied it to recording small voltages of short duration, which is the category into which heart-signals fall. His recording techniques have not been improved upon very much since they were first published many years ago. Here it should be noted that the term "lead" as used herein is being used in the medical sense and not the electronic sense (i.e., "lead" is a spatial position at which the heart-signal is viewed, not a wire).
After Einthoven's work, the entire field of research stagnated for nearly thirty years until the introduction by Wilson of upper and lower extremities' local leads and the zero electrode used in unipolar recordings.
The entire twelve-lead system is fed by unipolar and bipolar signals. Unipolar leads are divided into unipolar extremity or limb leads and unipolar precordial or chest leads.
In the unipolar lead system, the limb leads are:
aVR--the unipolar right arm lead, (R designating the right arm); PA1 aVL--unipolar left arm lead, (L designating the left arm); and PA1 aVF--unipolar left leg lead. PA1 1. Rhythm PA1 2. Rate PA1 3. P wave PA1 4. P-R interval PA1 5. QRS interval PA1 6. QRS complex PA1 7. ST segment PA1 8. T wave PA1 9. U wave PA1 10. Q-T duration
In all of these limb leads, the "a" stands for "augmented". The unipolar chest leads are designated by the letter "V" followed by a subscript numeral which represents the exact location on the chest. In a standard electrocardiographic setting there are six precordial leads, V.sub.1 -V.sub.6.
In the unipolar lead system, the potential differences are measured between each of the electrodes that are placed on the right arm, left arm, left leg, and precordial points V.sub.1 -V.sub.6 on the chest and a common reference point consisting of an electrode on the right leg. Each of the lead electrodes is independently considered as an active point compared to the common reference electrode (point) on the right leg, and is measured in relation to that common reference electrode.
In standard bipolar leads, lead I is the potential difference between the arms, i.e., the left arm potential minus the right arm potential. Lead II is the potential difference between the left leg potential and the right arm potential. Lead III is the potential difference between the left leg and the left arm. If the leads are diagrammed on the body they inscribe, essentially, an equilateral triangle. The polarity of these widely-separated bipoles was arbitrarily determined many years ago in order to record upright electrical deflections in these three limbs leads in most normal objects. The electrocardiograph generates the lead voltages from the potentials applied to it from the electrodes. The term "lead" as used in electrocardiography means view of the heart's electrical impulse. That view varies among leads.
The electrocardiograph is widely used by the medical profession. The standard electrocardiograph requires at least ten wires which are attached to the body of the patient at one end and to the electrocardiograph at the other end to detect heart-signals and transform them into a twelve-lead electrocardiogram evaluation. This involves attaching six electrodes to the chest or precordial area to obtain recordings of leads V.sub.1 -V.sub.6 as well as attaching four electrodes to the arms and legs of the patient to obtain recordings of leads I, II, III, aVR, aVL and aVF. (For heart rhythm monitoring, only three electrodes and three terminal wires are applied to the chest.) After the ten electrodes are attached to the patient, ten specific wires must be connected between each specific electrocardiograph terminal and the related electrode of the predetermined position.
In electrocardiographic terminology, the terms "dipole", "bipole" and "unipolar" have different meanings and applications. The "single dipole" concept is used to represent the local spread of excitation over cardiac tissue as recorded by a single recording electrode. This local excitation is in the form of a local influx and/or outflux of electrically charged elements, referred to as ions, through the cell membrane. The term "equivalent dipole" has been a term used since the days of Einthoven to represent the theoretical "electrical center" of a volume conductor used to describe the progression, magnitude and location of the electrical activity of the human body. This "equivalent dipole" has both direction and magnitude at any instance in the cardiac cycle and is traditionally represented as a vector that points in the direction of the positive pole of a dipole having both positive and negative poles. The vector has a length proportional to the magnitude of the dipoles' potential difference (i.e., the potential difference between its positive and negative poles).
The term "bipolar" has several uses in clinical electrocardiography and electrophysiology. Bipolar endocardial and epicardial recordings refer to recordings made between a cathode and anode of a recording device which are relatively closely spaced (e.g., several millimeters to one centimeter). For example, bipolar cardiac recordings are taken by modern pacemakers having leads that are reasonably closely spaced. In surface electrocardiographic practice, bipolar lead systems, as discussed above, are defined as limb lead systems that measure the potential differences between the three limb electrodes on the right and left arms and the left leg. The term "unipolar" is used in the practice of surface electrocardiography as described above.
The conventional and currently existing electrocardiographic systems are limited in operation. An early manifestation of acute myocardial ischemia is the development of ST-segment and T-wave changes. Clinical decisions for treatment are based on ST-segment shifts on the surface electrocardiogram. ST-segment depression is believed to represent subendocardial involvement, with less extensive myocardial injury. ST-segment elevation reflects transmural involvement, with greater extent of myocardial injury. Currently existing electrocardiographic monitoring equipment in the coronary intensive care units (CICU) and intensive care units (ICU) provides single-lead arrhythmia monitoring of cardiac events which is unable to detect myocardial ischemia in real time occurrence.
In the surgical setting, the cardiac catheterization laboratory protocol of percutaneous transluminal coronary angioplasty (PTCA) procedures employs the use of three-lead arrhythmia monitoring which is unable to detect ischemic events during actual performance of percutaneous transluminal coronary angioplasty.
In the ambulatory setting, Holter monitoring provides only arrhythmia recording and detection, which is unable to identify or locate coronary silent ischemia. Transtelephonic electrocardiogram transmission currently employs single-lead arrhythmia monitoring which is unable to identify or locate myocardial ischemic events in patients who have undergone percutaneous transluminal coronary angioplasty procedures, coronary artery bypass graft (CABG) surgery, or are currently being treated with antiarrhythmic drugs or are experiencing stable angina pectoris episodes.
In other settings, existing protocols employ single-lead electrocardiographic monitoring in the coronary intensive care mobil unit and emergency room, thereby permitting arrhythmia monitoring only. As can be seen, current coronary care electrocardiographic monitoring techniques are aimed at detection of cardiac arrhythmias rather than myocardial ischemia.
Existing electrocardiographic systems are also limited in diagnosing myocardial ischemia after noncardiac surgery. Patients undergoing noncardiac surgery sometimes have postoperative cardiac events. Adverse cardiac events are a major cause of morbidity and mortality after such surgery. It is necessary to determine the predictors of these outcomes in order to focus efforts on prevention and treatment. It would be helpful to know which patients are at highest risk. Clinical experience has demonstrated that postoperative myocardial ischemia during the first 48 hours after surgery confers a nearly threefold increase in the odds of having an adverse cardiac outcome and, more importantly, a ninefold increase in the odds of having an ischemic event (cardiac death, nonfatal myocardial infarction or unstable angina) in patients undergoing noncardiac surgery. In some clinical studies, postoperative myocardial ischemia was prevalent, occurring in more than 40 percent of the patients, and was silent in nearly all cases studied.
In addition, many and frequent difficulties are associated with the practical operation of the conventional and currently existing electrocardiographic systems due to the following factors:
1. The need to connect predetermined specific wires to predetermined specific electrodes (e.g. defined limb and side to defined wire, as well as specific precordial points to defined precordial wires) is time-consuming. In addition, connection errors are relatively frequent.
2. The wires often need to be untangled, resulting in the loss of precious time.
3. Existing electrocardiographic systems are somewhat impractical for use in coronary intensive care mobile units where speed of operation is critical.
4. Wire defects and damage are difficult to detect.
5. During many surgical procedures, single lead arrhythmia monitoring wires extend beneath the sterile surgical field. These wires often become disconnected from the electrodes and can interrupt the surgical procedure. In addition, existing electrocardiographic systems do not permit myocardial ischemia detection during surgery.
6. Patients in intermediate coronary care units sometimes disconnect the signal carrying wires from the electrocardiographic monitor while ambulating. By doing so, cardiac rhythm monitoring is interrupted.
7. Current electrocardiographic monitoring is limited in range and distance by the proximity between the patient and the electrocardiograph or monitor.
8. Current percutaneous transluminal coronary angioplasty (PTCA), diagnostic heart catheterization and other invasive interventional procedures performed in the cardiac catheterization suite; electrocardiographic monitorings in coronary intensive care units, intensive care unit (ICU) and coronary intensive care mobile units; and thrombolytic therapy monitoring; in each case, employ single lead or three lead electrocardiographic detection which provides only arrhythmia monitoring and is unable to diagnose myocardial ischemic events.
9. Patient compliance with the procedures and requirements of current electrocardiographic systems is minimal.
Therefore, it is an object of this invention to overcome the problems previously experienced in connection with the application of electrocardiographs in the taking of electrocardiograms and in connection with the rhythm monitoring of patients.
Another object of this invention is to provide an electrocardiographic and monitoring system in which the physical wires between the patient and the electrocardiograph or monitor are eliminated.
Another object of this invention is to provide an electrocardiographic and monitoring system in which a reduced standard number of wireless electrodes provide a complete standard twelve-lead electrocardiogram.
Another object of the present invention is to provide a precordial strip assembly containing a plurality of conductive elements for placement on the precordium area of a patient.
Another object of the invention is to provide a precordial strip assembly containing a reference conductive element permitting elimination of the standard right leg reference electrode.
Another object of the invention is to provide a precordial strip assembly having RA and LL conductive elements positionable on the patient in a position remote from the V.sub.1 through V.sub.6 and LA conductive elements.
Another object of the invention is to provide a self contained precordial strip assembly for detecting and transmitting heart signals.